The present invention relates in general to the field of bottom-to-surface type connections of the type including a vertical underwater pipe referred to as an up column or riser, connecting the sea bed to a floating support installed on the surface.
Once the depth of water becomes large, production fields and in particular oil fields are generally operated from floating supports. In general, the floating support has anchor means to keep it in position in spite of the effects of currents, winds, and swell. It generally also includes oil storage and processing means together with means for off-loading to off-loading tankers. Such tankers arrive at regular intervals to off-load production. Such floating supports, which are generally of the barge type, are commonly referred to by the initials xe2x80x9cFPSOxe2x80x9d for floating production storage off-loading. Numerous variants have been developed such as SPARs (which are long vertical floating cigar-shaped objects held in position by catenary anchoring) or indeed TLPs (tension leg platforms) which are platforms whose xe2x80x9clegsxe2x80x9d are anchor lines under tension, which lines generally extend vertically.
Wellheads are often distributed over an entire field and production pipes, together with water injection lines and inspection and control cables, are all placed on the sea bed to extend towards a fixed location vertically below the position of the floating support on the surface.
Some wells are thus situated vertically beneath the floating support and the insides of such wells can then be accessed directly from the surface. Under such circumstances, a wellhead fitted with its xe2x80x9cChristmas treexe2x80x9d can be installed on the surface on board the floating support. It is then possible to use a derrick installed on said floating support to perform all of the drilling, production, and maintenance operations required on the well throughout its lifetime. This is referred to as a xe2x80x9cdryxe2x80x9d wellhead.
With SPARs and TLPs, dry wellheads are brought together in a limited zone of the platform over which the derrick is displaceable to take up a position vertically over each of the wells so as to perform drilling operations or maintenance operations on a well throughout the lifetime of the field.
Since a drilling derrick is a tall structure, possibly 60 meters (m) tall and possessing a hoisting capacity that can exceed 500 tonnes (t), the means required for moving it from one well to another and for keeping it in position during operations on any particular well give rise to structures that are complex and expensive.
In order to maintain the riser fitted with its dry wellhead in a substantially vertical position, it is appropriate to exert upward traction which can be applied either by a cable tensioning system using winches or hydraulic actuators installed on the floating support, or else by means of floats distributed along the riser and installed at various depths, or indeed by a combination of those two techniques.
French patent FR 2 754 011 describes a barge and a guidance system for a riser, in which the riser is fitted with floats.
SPARs and TLPs are likewise fitted with a multiplicity of risers supported by floats that are held in position by guidance systems.
In FPSOS, risers reach the surface in a central cavity of the floating support that is referred to as a xe2x80x9cwellbayxe2x80x9d. The cavity passes right through the hull vertically over a height of about 30 meters, drawing water to a depth of about 20 meters. It is generally installed on the axis of the floating support, at equal distances from its ends since this is the zone in which the amplitudes of movements and of accelerations are the smallest when the vessel is subjected to the phenomena of roll, pitch, and yaw.
The depth of water over some oil fields exceeds 1500 m, and can be as much as 2000 m to 3000 m, so the weight of risers for such depths requires vertical forces to be deployed to keep them in position that can be as great as or more than several hundreds of tonnes. Buoyancy elements of the xe2x80x9ccanxe2x80x9d type are used which are installed at various levels along risers connecting the surface to ultragreat depths (1000 m-3000 m).
The floats concerned are of large dimensions and in particular they have a diameter in excess of 5 m, and a length of 10 m to 20 m, with each float delivering buoyancy that can be as much as 100 tonnes.
The float and the riser are subjected to the effects of swell, and of current, but since they are connected to the FPSO on the surface, they are also indirectly subjected to the effects of wind. This gives rise to lateral and vertical movements of the assembly comprising the riser, the floats, and the barge, which movements can be large, as much as several meters, particularly in a zone that is subject to swell.
To ensure that risers do not interfere with one another and do not interfere with the hull of the floating support, said risers are spaced apart from one another by several meters and also by several meters from the walls of the wellbay, which means that such wellbays can often be as much as 80 m long and 20 m wide on FPSO barges that are themselves as much as 350 m and 80 m wide, and rising by as much as 35 m above the water line. Such barges have a deadweight that can be as much as or greater than 500,000 tonnes deadweight.
These riser movements give rise to large differential forces between a riser and the guidance systems secured to the floating support.
The amplitude of the displacements and the very high level of the forces in the risers make it necessary to design guidance systems capable of withstanding not only extreme conditions, but also phenomena of fatigue and wear of the kind that can accumulate over the lifetime of such an installation, which can exceed 25 years.
The present invention relates to guiding risers within the generally central cavity when in a production position or when in a position in which drilling operations and heavy maintenance operations can be performed on the well, i.e. using a derrick which is fixed relative to the floating support, or indeed on the means for transferring the riser between these various positions.
The present invention also relates to transferring risers within the generally central cavity between their production positions and a position in which drilling operations and heavy maintenance operations can be performed on the well, i.e. using a derrick which is fixed relative to the floating support.
Well drilling is thus performed on the main axis of the derrick through a xe2x80x9cdrillingxe2x80x9d riser whose function is to guide the drill strings and to contain the mud returned from a well that is being drilled. Such a drilling riser is made up from unit lengths that can be as much as 50 m long, with the entire assembly being lowered step by step as the said riser is assembled. At the end of drilling, the portion of the riser corresponding to the depth of water is disconnected from the well at the sea bed, and is then taken to a parking position after its length has been shortened by removing one or two of the unit lengths. By proceeding in this way, the drilling riser remains suspended with its bottom end situated at 50 m to 100 m from the sea bed.
The production riser can then be assembled step by step in the same manner until it reaches the entrance to the well. Floats are installed on the top portion as it is put into place, and finally the bottom of the production riser is connected to the well. The well is then fitted with various items of production tubing and the xe2x80x9cChristmas treexe2x80x9d of the dry wellhead is put into place.
The assembly is then transferred to its xe2x80x9cslotxe2x80x9d, i.e. its production position in which it will remain throughout the lifetime of the field, except when certain maintenance operations are performed that require said riser to be returned to the main axis of the derrick in order to perform heavy maintenance operations.
Means are known for guiding a riser relative to a floating support. However, those guide systems usually imply cable-tensioning systems which are difficult to implement (GB 2 170 240 or U.S. Pat. No. 4,231,429). Other systems have been proposed that are not general purpose but are specific to the riser being located in one position relative to the floating support, and in particular on the axis of the derrick.
Cable-tensioning guidance systems make it lengthy and difficult to transfer a riser from one position to another within the central cavity or wellbay, particularly when displacing a riser from its production position to being in position on the axis of the derrick.
An object of the present invention is to provide apparatus for holding and guiding a riser, in particular a floating riser, relative to the floating support, which apparatus is simple to implement and can be used whatever the position of the riser, in particular when the riser is in its production position or in its drilling position or maintenance position on the axis of the derrick; and even while the riser is being transferred from its production position to the axis of the derrick.
The apparatus must allow relative movement to take place between the floating support and the riser so as to withstand common loading forces corresponding to lateral loads that can be as great as 10 tonnes, and occasional extreme forces corresponding to loads that can be as great at 100 tonnes or even 200 tonnes.
The device must not only allow substantially vertical displacements of up to 5 meters for relative movements between the floating support and the risers in normal operation, but it must also accept extreme displacements corresponding to the floating support being fully deballasted or to being fully ballasted, which corresponds to a total stroke that can be as much as 15 meters to 20 meters. In addition, for operations performed on the axis of the derrick, the riser must remain substantially in line with the axis of the derrick.
Another object of the present invention is to provide guidance apparatus enabling the riser to be transferred from one position to another within the central cavity or wellbay in a manner that is simple to perform and can therefore be performed in complete safety.
To do this, the present invention provides apparatus for holding and guiding a riser, in particular a floating riser, relative to a floating support, the apparatus having guide means enabling said riser to slide along its longitudinal axis and guiding lateral displacements thereof in a horizontal plane perpendicular to said longitudinal axis of the riser. The apparatus of the invention comprises joint means secured to said floating support and enabling:
a) said riser to rotate about a horizontal axis perpendicular to the longitudinal axis of said riser within a cone of half-angle at the apex that is less than or equal to 10xc2x0, said horizontal axis and said apex of the cone being situated substantially at the center of the riser and level with the midplane of the zone in which said joint means are situated along the longitudinal axis; and
b) said riser to slide along its longitudinal axis and for the lateral displacements of said riser to be guided in a plane substantially perpendicular to said longitudinal axis of the riser; and
said joint means comprising:
first friction pads mounted on a pad support enabling said pads to pivot about an axis perpendicular to said longitudinal axis of said riser; and
said first pads co-operating with second friction pads, or preferably with wheels, so that said second pads or said wheels bear against said riser and enable it to slide, and said riser bears against said first pads only when said wheels are displaced under the effect of lateral displacements of said riser. It will be understood that the lateral displacements of the riser are generated by relative forces between the floating support and the riser.
The term xe2x80x9clongitudinal axis of the riserxe2x80x9d is used to mean the vertical axis when the riser is in its rest position, i.e. when it is not subject to movement associated with movements of the sea.
The apparatus of the invention is designed to support varying amounts of force in the horizontal plane. Said second pads or the wheels preferably serve to guide the riser when subjected to everyday forces at low loadings of up to about 10 tonnes and the first pads provide guidance when the riser is subjected to heavy loadings under extreme conditions, in particular loadings up to 100 tonnes or even 200 tonnes.
Said first pads have respective bearing surfaces on said riser which are preferably curved in a manner that is complementary to the curve of the riser.
In an embodiment, said first pads can pivot about respective axes secured to said pad support.
In another embodiment, said pads are fixed relative to said pad support and said pad support can pivot relative to said floating support to enable said pivoting of said first pads about an axis perpendicular to said longitudinal axis of said riser.
More particularly, said first pads are fixed on a tubular element encasing said riser coaxially, said tubular element being mounted on a ball and socket joint enabling said pivoting and said rotation respectively of said pads and of said riser.
In the apparatus of the invention, said ball and socket joint can be a laminated abutment with said tubular element being embedded therein.
Advantageously, said second pads or preferably said wheels are mounted on axes perpendicular to the vertical axis of said riser, the axes of said second pads or preferably said wheels being capable of moving in translation along axes that are perpendicular to the longitudinal axis of said riser.
Said first pads are preferably distributed symmetrically around said riser and comprise at least three pads, preferably all situated at the same level along the longitudinal axis of said riser.
Said second pads or preferably said wheels are preferably disposed symmetrically around said riser about its longitudinal axis and with at least three wheels having axes of rotation that are preferably situated substantially in a single plane.
In an embodiment, said first pads are mounted on an axis that is situated substantially in the same horizontal plane as the axes of said wheels and said first pads, and said wheels are disposed successively and symmetrically around said riser in alternating manner.
In another embodiment, said first pads and said wheels are disposed in a staggered configuration, with their respective pivot axes and rotation axes being situated at different respective levels along the longitudinal axis of said riser.
More particularly, said first pads are disposed on at least two different levels along said longitudinal axis of said riser, and said levels are disposed symmetrically about a horizontal plane corresponding substantially to the midplane of the zone in which said joint means are situated along the longitudinal axis.
In another variant embodiment, the axes of rotation of the wheels are disposed on at least two different planes perpendicular to said longitudinal axis of said riser, and said planes are disposed symmetrically about a horizontal plane corresponding substantially to said midplane of the zone along the longitudinal axis of said riser where said joint means are situated.
In another variant embodiment, said first pads can be situated on two different levels along the longitudinal axis of said riser, an upper level and a lower level, and said wheels are situated above said upper level and below said lower level.
In another variant, said first pads are situated on two different levels, an upper level and a lower level, and said wheels are situated between said levels.
The holding and guidance apparatus of the invention can be installed on the floating support to hold and guide one of said risers in its production position, or it can be connected to transfer means for transferring one of said risers from its production position to the axis of a drilling derrick, in both cases, said joint means are preferably disposed level with the junction floor between said riser and said support, situated between the deck of the floating support and water level.
Also preferably, the apex of the cone whose halfangle at the apex is less than 10xc2x0 is situated substantially at the level of said junction floor.
Holding and guidance apparatus of the invention can also be installed on said floating support to hold and guide one of said risers in its operating position in a derrick installed on said floating support, and more precisely on the axis of said derrick.
Holding and guidance apparatus of the invention can be located on the axis of the derrick at various levels along the longitudinal axis of the riser ZZxe2x80x2. Depending on its position along the longitudinal axis, the dimensioning of the pads and the wheels needs to be adapted to match the forces that are to be transmitted and the relative movements of the riser that can be accommodated relative to the floating support.
It is possible to begin by putting the apparatus of the invention in place at the junction floor between the riser and the floating support, i.e. in a position that is intermediate between the deck of the floating support and water level. Under such circumstances, it is possible to use the same type of apparatus in the production position and while the riser is being transferred from its production position to the axis of the derrick. The wheels or the second pads are then dimensioned to support ordinary forces corresponding to loads that can reach 5 tonnes, and said first pads are dimensioned to support extreme forces corresponding to loads that can be as much as 50 tonnes.
It is also possible to position apparatus of the invention at at least one of two levels Pxe2x80x21 and Pxe2x80x22, preferably respectively above and below said junction floor between the riser and the floating support, the levels Pxe2x80x21 and Pxe2x80x22 being situated at different levels of the wall of the cavity in the floating support, and being sufficiently spaced apart to hold said riser substantially on the axis of the derrick, and to ensure that lateral displacements of the riser do not enable its axis to depart by more than 5xc2x0 from the axis of the derrick, and preferably by no more than 2xc2x0.
The present invention thus provides apparatus including said joint means comprising said first pads cooperating preferably with wheels situated on a first level Pxe2x80x21 and on a second level Pxe2x80x22 along the wall of the central cavity of the floating support, respectively above and below the level P0 of said junction floor between the riser and said floating support.
To limit ordinary forces and extreme forces transmitted to the planes Pxe2x80x21 and Pxe2x80x22 to loads respectively of 10 tonnes and of 100 tonnes, it is advantageous for the planes Pxe2x80x21 and Pxe2x80x22 to be spaced apart as much as possible, in particular by at least 5 meters, and preferably by at least 10 meters.
Insofar as the height of the wall of the cavity in the floating support is about 30 meters, given that it draws about 10 meters of water, and insofar as the plane Pxe2x80x21 is situated above water level and slightly below the level of the deck of the floating support, it is possible for the lower plane Pxe2x80x22 to be situated below water level.
In this position on the axis of the derrick, said joint means can be fixed inside a collar which is held in place around the riser by cables connected to a tensioning system to the wall of the cavity of said floating support, at least at one of said levels Pxe2x80x21 and Pxe2x80x22.
In another embodiment, said joint means comprising said first pads preferably co-operating with said wheels are situated at the end of a system of articulated arms including hydraulic actuators.
Advantageously, the system of articulated arms can move between a closed position and an open position such that in the closed position, said joint means preferably comprise said wheels which are in contact with said riser, and in the open position the system allows a float fitted to said floating riser to pass through, said joint means and preferably said wheels remaining in contact with said float.
Advantageously, each of the articulated arms in a given plane Pxe2x80x21 or Pxe2x80x22 can be adjusted in length, thereby making it possible to establish an offset of known value between the axis of the riser and the axis of the derrick.
The present invention also provides a method of holding and guiding a floating riser relative to a floating support, the method enabling said riser to slide along its longitudinal axis ZZxe2x80x2 and guiding its lateral displacements in a horizontal plane XXxe2x80x2, YYxe2x80x2 perpendicular to said riser by means of at least one holding and guidance apparatus of the invention.
The present invention also provides a method of transferring a riser on a floating support from a production position to a position within a drilling derrick. In the present invention, said riser is moved in said cavity of a floating support by displacing said holding and guidance apparatus of the riser relative to the floating support of the invention, itself secured to said riser.
Advantageously, the floating support has a central cavity with a plurality of compartments at the ends of which said risers are in their production positions, said compartments communicating with a central channel, at the end of which there is situated a drilling derrick, said compartments extending transversely relative to said central channel.
Advantageously, said compartments and said central channel comprise a junction floor between said riser and said floating support on each of the rims defining a continuous channel of substantially constant width.
In a particular embodiment, said junction floor is secured to the wall of the cavity at a level that is intermediate between the deck of the floating support and water level.
The floating support preferably comprises means for transferring said risers between their production positions at the ends of the compartments, and the derrick position, said transfer means enabling one of said risers to be moved along said junction floor in cooperation with said holding and guidance apparatus that provides the junction between said riser and said floating support at said junction floor.
In a variant embodiment, said transfer means are fixed and secured to said floating support.
In particular, said transfer means comprise a set of winches and cables connecting said winches to said riser.
Advantageously, said transfer means comprise a carriage that is movable along said compartments and said channel.
Also advantageously, said junction floor is fitted with guide rails which enable said transfer means or said riser, in particular if said transfer means are fixed, to move along said compartments and said central channel.